The Prandtl–Glauert singularity is a theoretical construct in flow physics, often incorrectly used to explain vapor cones in transonic flows. It is the prediction by the Prandtl–Glauert transformation that infinite pressures would be experienced by an aircraft as it approaches the speed of sound. Because it is invalid to apply the transformation at these speeds, the predicted singularity does not emerge. The incorrect association is related to the early-20th-century misconception of the impenetrability of the sound barrier.
The Prandtl–Glauert transformation assumes linearity. This assumption becomes inaccurate at high Mach numbers and is entirely invalid in places where the flow reaches supersonic speeds, since sonic shock waves are instantaneous changes in the flow. Indeed, one assumption in the Prandtl–Glauert transformation is approximately constant Mach number throughout the flow, and the increasing slope in the transformation indicates that very small changes will have a very strong effect at higher Mach numbers, thus violating the assumption, which breaks down entirely at the speed of sound. This means that the singularityfeatured by the transformation near the sonic speed is not within the area of validity. The aerodynamic forces are calculated to approach infinity at the so-called Prandtl–Glauert singularity; in reality, the aerodynamic and thermodynamic perturbations do get amplified strongly near the sonic speed, but they remain finite and a singularity does not occur. The Prandtl–Glauert transformation is a linearized approximation of compressible, inviscid potential flow. As the flow approaches sonic speed, the nonlinear phenomena dominate within the flow, which this transformation completely ignores for the sake of simplicity.
This formula is known as "Prandtl's rule", and works well up to low-transonic Mach numbers . However, note the limit: This obviously nonphysical result is known as the Prandtl–Glauert singularity.
Reason for condensation clouds
The reason that observable clouds sometimes form around high speed aircraft is that humid air is entering low-pressure regions, which also reduces local density and temperature sufficiently to cause water to supersaturate around the aircraft and to condense in the air, thus creating clouds. The clouds vanish as soon as the pressure increases again to ambient levels. In the case of objects at transonic speeds, the local pressure increase happens at the shock wave location. Condensation in free flow does not require supersonic flow. Given sufficiently high humidity, condensation clouds can be produced in purely subsonic flow over wings, or in the cores of wing tips, and even within, or around vortices themselves. This can often be observed during humid days on aircraft approaching or departing airports.
